binman: Support adding a U-Boot environment

In some cases it is useful to include a U-Boot environment region in an
image. This allows the board to start up with an environment ready to go.

Add a new entry type for this. The input is a text file containing the
environment entries, one per line, in the format:

   var=value

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>